Crystal oscillators are widely used to provide accurate frequency references in wireless and wire-line transceivers. However, due to inaccuracies in crystal manufacturing, crystal aging effects, and temperature variations, finite frequency errors often exist between the actual and ideal target frequencies of the RF transmit carrier and local oscillator (LO) of the receiver.
In a wireless receiver, such frequency errors give rise to corresponding frequency shift in the IF frequency of the receiver when compared to the ideal IF frequency. For example, a ±50 ppm frequency error in a transmitter's crystal reference frequency will result in a ±50 kHz frequency error in an RF carrier frequency at a typical transmit frequency of about 1 GHz. Ignoring any errors in the receiver's crystal frequency, this causes a corresponding shift of ±50 kHz in the IF frequency of the receiver. The frequency errors in the receiver require additional bandwidth to be allocated to the IF filter of the receiver to ensure the filter will not attenuate the wanted spectrum in the presence of the frequency errors. However, the excess IF bandwidth degrades the selectivity performance of the receiver and also causes excess noise at the output of the IF filter which reduces the sensitivity of the receivers. Moreover, the performance of the demodulator (e.g., an FSK demodulator) in the radio receiver is also dependent on IF frequency errors and, in general, for narrow-bandwidth transmissions, the output signal-to-noise ratio (SNR) of the demodulator of the receiver falls rapidly when frequency errors exceed a certain threshold.
Conventional frequency measurement systems can be used to measure the frequency of the modulated input signals, e.g., analog quadrature FSK modulated signals, input to the demodulator. One such conventional frequency measurement system employs quadri-correlators and differentiators to measure the frequency of the input signals. However, the system does not measure the center or IF frequency of the input signals. Moreover, the design relies on generating an output signal which is proportional to the square of the magnitude of the input signal. Hence, signal normalization and/or automatic gain control must be performed to remove the amplitudinal dependent term from the output signal. Signal normalization increases the design complexity and power requirements of the system.
One approach to reducing the frequency errors is to manually calibrate the crystal oscillator. This requires a one-time manual calibration of the crystal frequency that removes the nominal frequency errors in wireless transceivers. However, this approach adds significantly to the manufacturing cost of the radio module and does not address frequency shifts due to temperature variation or aging. Temperature controlled crystal oscillators have been also used to provide high accuracy, stable frequency references. However, these devices are expensive and not suitable for low cost wireless transceiver applications.
Another approach to reducing the frequency errors is to utilize an automatic frequency control system. However, conventional automatic frequency control (AFC) systems which attempt to control the frequency errors have complex designs which consume significant power and typically require expensive oscillator crystals and/or manual calibration. Conventional AFC systems do not remove or adjust the IF frequency errors in the receiver such that optimum IF frequency is utilized to maximize the selectivity and the SNR of the demodulator.